Grgic wrote:Is anyone able to tell me if the Shimano Ultegra 6750 Hollowtech II Compact Chainset will work on a Shimano BB – 4500 bottom bracket?

Yes.

There are only two types of HollowtechII external bearing BBs: road and MTB.

The MTB ones are 1mm narrower on each bearing race, longer in the thread, and are supposed to be used with spacers behind them. Road ones... the opposite to that.

BB-4500 is nominally a Tiagra BB, so it will be the wide-bearing-no-spacers road style. May or may not be any different in construction to higher specced road group BBs... may or may not be just a different colour for a different price .

Either way, a Tiagra HT2 BB will certainly fit an Ultegra HT2 crankset.

I'll leave it to others to decide on your behalf whether or not you really want an Ultegra HT2, because strangers on the internet know best

barefoot wrote:The MTB ones are 1mm narrower on each bearing race, longer in the thread, and are supposed to be used with spacers behind them.

The spacers are only to allow compatibility with various bottom bracket shell widths, and BB-mounted chain guides or front derailleurs or not. It's a case of design for the widest assembly necessary, and then add spacers as other bits get removed.

barefoot wrote:The MTB ones are 1mm narrower on each bearing race, longer in the thread, and are supposed to be used with spacers behind them.

The spacers are only to allow compatibility with various bottom bracket shell widths, and BB-mounted chain guides or front derailleurs or not. It's a case of design for the widest assembly necessary, and then add spacers as other bits get removed.

True - but the total installed width of a MTB BB + spacers + frame is wider than the installed width of a road BB + frame. The spindle of a MTB crankset is longer to suit.

Main point being, though - all road group BBs are the same; all MTB group BBs are different to that, and not readily [1] interchangeable with the road ones.

tim

[1] I have a MTB BB and road crankset and a spacer on my road bike. Oops.

I think you missed my point a bit Tim.... MTB bottom bracket shells are commonly 73mm wide, compared to 68 (and increadingly rarely 70) mm. Then factor in the use of a BB-mounted chain device & you have an assembly that takes quite a bit of real estate that needs to be designed for. The spacers are to compensate for the absence of chain device, narrower BB shell or a combination of both.

Im sure you can run a mtb bb in a road bike set up not that you would really want to, most likely not too safe running a roadie bb in a mtb because the thread lengh is shorter unless you have a 68 mm bb frame with no brackets attached to the drive side

Dragster1 wrote:Im sure you can run a mtb bb in a road bike set up not that you would really want to...

Why not - it's common practice to use MTB cranksets with touring road bikes.

They use them on cyclocross too, The road bbs are smoother, The road bike frame bb may not except the longer thread of the drive side bearing housing and the chain line measurement can be different also depending of what bike you have.

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